1. Field of the Invention
The present invention relates to a transmission power control method for a mobile telephone communication, particularly in a code division multiple access (CDMA) system.
2. Description of the Related Art
CDMA technique is described in the literature titled xe2x80x9cCDMA Principles of Spread Spectrum Communicationxe2x80x9d authored by Andrew J. Viterbi, and published Addison Wesley Publishing Company.
In such CDMA, mobile stations use a carrier frequency in the same frequency band, and the transmission signal from each mobile station is identified by a spreading code uniquely assigned to each mobile station.
FIG. 2 shows the construction of a mobile station that works in a conventional transmission power control method.
Referring to FIG. 2, there are shown a transmitting/receiving antenna 11, a high-frequency module 12 for converting into a signal in a spread band a signal received from a base station, a rake receiver 13 for decoding the output signal of the high-frequency module 12 into a baseband signal, a transmission power control bit reproducer 14 for extracting a transmission power control bit which is sent by the base station at regular periods, a transmission power amplifier gain controller 15 for controlling the gain of a power amplifier, a high-frequency module 16 for converting a speech channel signal into a radio-frequency band signal, and the variable-gain amplifier 17 for amplifying the output signal of the high-frequency module 16 to transmit the amplified signal from the transmitting/receiving antenna 11. The gain of the variable-gain amplifier 17 is controlled by the transmission power amplifier gain controller 15.
To assure the quality of communication to all mobile stations and to treat all mobile stations in a fair way, the base station needs to have received powers at equal level from the mobile stations. The power of a signal received from a mobile station is subject to variations (fading) along with the movement of the mobile station.
In the conventional mobile station shown in FIG. 2, a base station reports to the mobile station that the received power is insufficient, and the mobile station adjusts its transmission power in accordance with the report. This control is thus a closed-loop transmission power control. The transmission power control is also provided with an open-loop control capability in which the mobile station automatically controls its transmission power with no instruction received from the base station.
The operation of a conventional mobile station is discussed.
The base station monitors the received power of a signal transmitted by the mobile station for a constant segment of time (Tpc (seconds)). The base station issues an instruction signal of 1 bit (transmission power control bit) for lowering the transmission power of the mobile station by a certain percentage if the average power during the constant segment is greater than a predetermined value, and issues an instruction signal of 1 bit (transmission power control bit) for raising the transmission power of the mobile station by a certain percentage when the average power during the constant segment is smaller than the predetermined value. These signals are transmitted to the mobile station over a downlink speech channel (from the base station to the mobile station) every a predetermined segment (Tpc (seconds)).
The mobile station demodulates the received signal at its rake receiver 13, extracts, at its transmission power control bit reproducer 14, the transmission power control bit every constant period of segment from the demodulated signal from the rake receiver 13, and outputs the transmission power control bit to its transmission power amplifier gain controller 15. The transmission power amplifier gain controller 15 controls the variable-gain amplifier 17 in order to raise the transmission power by a certain percentage when the transmission power control bit indicates an instruction of power increase, or in order to lower the transmission power by a certain percentage when the transmission control bit indicates an instruction of power reduction.
In the conventional transmission power control method, the rate of increase or reduction in the transmission power of the mobile station is constant (the rate of increase or reduction in the transmission power is now called a step size in the transmission power control), regardless of the varying rate of change over time in the received power with the mobile station moving at some travel speed or under the occurrence of fading. For example, when a large increase is required in the transmission power, no matching rate of increase results. With the mobile station moving at a high travel speed, a variation in the transmission power of a signal received from the mobile station becomes abrupt, and the base station suffers a large error from a desired value in the received signal level.
In a first aspect, the present invention relates to the transmission power control method in a radio communications system for communication between a base station and a mobile station, wherein the base station detects a received power of a signal transmitted by the mobile station and transmits, to the mobile station, a transmission power control instruction signal for controlling the transmission power of the mobile station, along with a transmission signal, at regular periods; the mobile station monitors the transmission power control instruction signal transmitted by the base station and changes the rate of modification in the transmission power, based on either a setting of the number of consecutive transmission power control instruction signals that give the same content or a setting of the number of consecutive transmission power control instruction signals that give different contents, one from another; and the mobile station controls the transmission power of a signal to be transmitted, based on the transmission power control instruction signal transmitted by the base station and the changed rate of modification in the transmission power.
In a second aspect, the present invention relates to a transmission power control method in a radio communications system for communication between a base station and a mobile station, wherein the base station detects a received power of a signal transmitted by the mobile station and transmits, to the mobile station, a transmission power control instruction signal for controlling the transmission power of the mobile station, along with a transmission signal, at regular periods; the mobile station estimates the transmission power control instruction signal, based on the transmission power control instruction signal transmitted by the base station and a current rate of modification in the transmission power; the mobile station monitors the estimated transmission power control instruction signal and changes the rate of modification in the transmission power, based on either a setting of the number of consecutive transmission power control instruction signals that give the same content or a setting of the number of consecutive transmission power control instruction signals that give different contents, one from another; and the mobile station controls the transmission power of a signal to be transmitted, based on the transmission power control instruction signal transmitted by the base station and the changed rate of modification in the transmission power.
In a third aspect, the present invention relates to a transmission power control method in a radio communications system for communication between a base station and a mobile station, wherein the base station detects a received power of a signal transmitted by the mobile station and transmits, to the mobile station, a transmission power control instruction signal for controlling the transmission power of the mobile station, along with a transmission signal, at regular periods; the mobile station estimates a Doppler frequency to which the received signal is subject to under fading, based on the transmission power control instruction signal transmitted by the base station and a current rate of modification in the transmission power; the mobile station changes a rate of modification in the transmission power, based on the estimated Doppler frequency and the transmission power control instruction signal; and the mobile station controls the transmission power of a signal to be transmitted, based on the transmission power control instruction signal transmitted by the base station and the changed rate of modification in the transmission power.